Vinyl halide polymers, in particular polyvinyl chloride, are commonly used in both plasticized and unplasticized forms. Making these forms often subjects the polymer to high temperatures during processing. The processing of vinyl halide polymers requires the addition of one or more stabilizers to prevent degradation of polyvinyl halide, as evidenced by gross color change or "burning". Burning is the term used to describe the darkening of the vinyl material from a light or clear to black color. This color change is the product of thermal decomposition through dehydrohalogenation. In addition, the polymer exhibits an inferior mechanical quality after dedydrohalogenation as evidenced by an increase in brittleness.
In addition to the degradative effects of heat, vinyl halide polymers are discolored on extended exposure to light as often occurs in outdoor applications.
In commercial practice, several types of stabilizers have been employed to avoid this degradation. Organotin stabilizers have been employed with satisfactory results. However, organotin stabilizers are costly, and the supply of tin assures that the price will continue to increase. Some toxicity problems are also present in the use of certain organotin stabilizers. A combination of barium and cadmium carboxylate and phenate stabilizers have been used, exhibiting good stabilization properties, but not without some problems. There are toxicity problems associated with cadmium stabilizers in both the production of the stabilizers and in the end use. In fact, recent government actions, both in the United States and abroad, indicate a movement toward decreasing the allowable workplace exposure level to cadmium. Antimony stabilizers such as antimony mercaptides have also been used to prevent degradation. Lead carboxylates have effected excellent results as stabilizers for vinyl halide resins. However, the end use toxicity and production level toxicity associated with lead compounds is well known.
Zinc and calcium carboxylate combinations have been employed as stabilizing agents. However, such combinations have not been free of difficulties. Most zinc compounds used either alone or in combination as stabilizers lead to rapid degradation or "burning" of polyvinyl halides after a very short period of good color. In order to achieve acceptable stabilizing performance from zinc-calcium combinations, many co-additives or co-stabilizers must be employed, thereby increasing the cost of the stabilizer systems.
Therefore, while the use of zinc compounds as stabilizing agents for vinyl halide polymers is desirable in order to avoid the toxicity problems inherent in barium-cadmium and lead stabilizers and the expense problems associated with organotin stabilizers, there has been no fully acceptable way to achieve long term stabilization with zinc compound stabilizers.
However, the use of zinc compounds as stabilizers for vinyl halide resins has many advantages. First, zinc is basically non-toxic, and therefore avoids the inherent toxicity problems associated with cadmium and lead stabilizers. There appear to be no toxicity problems associated with zinc compounds at either the production level or in the end use level.
Second, zinc compounds exhibit excellent stability against photodegradation. While tin mercaptides produce acceptable results as stabilizers, zinc compounds are far superior in their ability to stabilize polyvinyl halides against the degradative effects of light.
Third, zinc produces no staining problems that often occur with cadmium, lead, antimony, and tin stabilizers. Airborne sulfur compounds react with cadmium stabilizers to produce undesirable color.
Fourth, zinc metal is much less expensive than tin metal, and zinc is more available than either cadmium or tin. The future availability of zinc appears promising.
Accordingly, it is an objective of this invention to provide novel zinc compounds with activity as light and heat stabilizers for vinyl halide polymers.
It is a further objective of this invention to provide novel zinc compounds which avoid the rapid degradation of polyvinyl chloride after short periods of exposure to heat or extended exposure to light.
Yet, another objective of this invention is to provide novel metallic amino acid complexes which have stabilizing capabilities for vinyl halide polymers.
The method of accomplishing these and other objectives will become apparent in the following description of the invention.